Electronic Interfaces for Resonant Strain/Force Sensors

Ken Wojciechowski
(Professor Bernhard E. Boser)

The objective of this research is to develop electronic interface circuits to measure strain with a silicon micromachined resonant sensor. These sensors are analogous to a guitar string. When the sensor is stretched (tensile strain) or compressed, its resonant frequency increases or decreases accordingly. Resonant sensors have several attributes that make them attractive. First, the information we want is contained in their output frequency and therefore sensor output is immune to AM noise. Second, their sensitivity to applied strains has been shown to be quite high [1].

While resonate sensors have the promise of high sensitivity, challenges remain in the development of the sensors. In fact, there are two components of the design that need to be improved from the current state of the art. The first component is the actual resonant sensor/oscillator, which is composed of a micromachined resonator and oscillator circuitry. The micromachined resonators have a high Q, but they suffer from high motional resistance [2]. This makes it difficult to make an oscillator, as we must match the impedance of the resonator with an equivalent negative resistance for oscillation to occur. The impedance match is performed by the oscillator circuitry. Improvements in design of the resonator and oscillator circuitry can significantly improve the linearity and phase noise of the oscillator, resulting in better sensor resolution. The second component is the method used to measure the sensor's output. In many applications, the change in resonant frequency is measured by frequency counting [3]. With this method, high accuracy measurements can be obtained, but at the expense of bandwidth. Another method of measurement uses FM demodulation to measure the change in frequency at the sensor output and is usually done with a PLL. With this method, better bandwidth is achieved; however, the phase noise of the VCO and the resonant oscillator make the DC measurement resolution quite poor.

This research will focus mainly on methods to improve oscillator circuit design and frequency measurement techniques to yield good sensor resolution over a large bandwidth.

J. J. Sniegowski, H. Guckel, and T. R. Christenson, "Performance Characteristics of Second Generation Polysilicon Resonating Beam Force Transducers," Technical Digest IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, June 1990.
C. T.-C. Nguyen, "Frequency-Selective MEMS for Miniaturized Low-Power Communication Devices," IEEE Trans. Microwave Theory and Techniques, Vol. 47, No. 8, August 1999.
T. A. Roessig, "Integrated MEMS Tuning Fork Oscillators for Sensor Applications," PhD dissertation, UC Berkeley, 1998.

Send mail to the author : (kenwoj@eecs.berkeley.edu)

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